What We Can Learn from the Running of the Spectral Index if no Tensors are Detected in the Cosmic Microwave Background Anisotropy

TL;DR

This paper explores how the measurement of the spectral index's running in the cosmic microwave background can reveal the presence of multiple scalar fields during inflation, especially if no tensor modes are detected.

Contribution

It demonstrates that in multi-field inflation models, the running of the spectral index can be significant and correlated with non-Gaussianity, unlike in single-field models.

Findings

Running can be large in multi-field models

Running correlates with non-Gaussianity

Detection implies multiple scalar fields during inflation

Abstract

In this paper we operate under the assumption that no tensors from inflation will be measured in the future by the dedicated experiments and argue that, while for single-field slow-roll models of inflation the running of the spectral index will be hard to be detected, in multi-field models the running can be large due to its strong correlation with non-Gaussianity. A detection of the running might therefore be related to the presence of more than one active scalar degree of freedom during inflation.